Unity custom Audio Manager won't work with custom Stop function

Hello there, I made an Audio Manager using Brackey's tutorial. The Play() function works perfectly, but here's how my music is set up.

From the start, I have a violin at the background. Whenever I get into combat I want this violin to stop, and the action music to start. However, whenever I want to start the game, I get a NullReferenceException and can't manage to figure out what the problem is, because I have an if (fadePlaying != null) before the whole thing works in the Update. This error makes the whole function not work (I think). The scripts:

Do you have an AudioManager in your scene? If not, FindObjectOfType() will return a null, and you never check for that. In general, always check the result of FindObjectOfType(), GetComponent(), and similar calls, because there might be nothing that fits your search.

Btw, your AudioManager class is a singleton so that you don't need to look for it, you can use AudioManager.instance to access it from anywhere (after a null check, of course ;) ).

If this is not the issue, then you should also check if Array.Find() in AudioManager.Play() return anything... maybe the audio files have different names than what you think (Unity usually cuts off the file extension).

Then it's debugging time! Either add null checks everywhere and print Debug.Log("... is null!"); if it is null, or if using Visual Studio, add breakpoints everywhere, especially in Awake() and Start() methods, and start in debug mode stepping through until you find what exactly is null.
Sorry, with this much info this is all I can offer.